My assumption when I first heard the name Amenta was that it was from the Latin word mens, mentis, meaning “mind.” I think I read somewhere that this was incorrect, but I could be misremembering and I can’t find that post.
Anyways, under that system, Mentan would mean something to the effect of “having a mind” and Amentan would mean something to the effect of “not having a mind.”
The opposite of American is:
un-American (the hyphen is unavoidable, don’t even try)
unAmerican (programmer’s CamelCase has infected the wider language)
unamerican (because fuck caps altogether)
Unamerican (as in from Unamerica, because the “adjective of” and “opposite of” operations commute)